num = input()
if len(num)==4:
  n1 = num[0]
  n2 = num[1]
  n3 = num[2]
  n4 = num[3]
  spis = [int(n1), int(n1+n2), int(n1+n2+n3), int(n2), int(n2+n3), int(n2+n3+n4), int(n3), int(n3+n4), int(n4), int(num)]
  spis.sort(reverse=1)
  for x in spis:
    F = 1
    for d in range(2, x//2+1):
      if x % d == 0:
        F = 0
        break
    if F==1:
      print(x)
      break
  if F==0:
    print('0')
else: print('0')
